1. Field of the invention
The invention relates to a rotation detecting device for a brushless motor.
2. Related art
Some electric motors such as a motor for driving a rotary cylinder head of a videotape recorder (VTR), and that for driving a rotary polygon mirror of an optical scanning device are provided with a frequency generator (FG) so that the rotating speed of the driving motor is detected and the detected speed is used in the speed control.
Generally, such a frequency generator attached to a motor or the like comprises a magnetized portion for the frequency generator (hereinafter, referred to as "FG magnetized portion") which is rotated together with a rotating body such as a rotor of the motor and magnetized and arranged in the circumferential direction, and a generating coil pattern (hereinafter, referred to as "FG pattern") which opposes the FG magnetized portion.
The FG pattern is formed on a circuit board so as to have a substantially circular ring shape, and comprises a plurality of generating lines which are radially elongated, and connecting lines which connect the generating lines so as to constitute a series connection unit.
A predetermined wiring pattern is formed on the circuit board.
FIGS. 4 to 6 show an example of a conventional brushless motor.
In FIGS. 4 and 5, the reference numeral 1 designates a rotation shaft. As shown in FIG. 4, the rotation shaft 1 is rotatably supported through a bearing 4 by the bearing holder 3 which stands on a circuit board (printed-circuit board) 2. A rotor case 5 which is rotated integrally with the rotation shaft 1 and has a substantially cup-like shape is fixed to the upper end portion of the rotation shaft 1 in FIG. 4. A ring-like rotor magnet 6 is disposed on the inner peripheral face of the outer flange portion of the rotor case 5. As shown in FIGS. 4 and 6, the rotor magnet 6 comprises a driving magnet pole portion 6a in which N-poles and S-poles are alternatingly formed in the circumferential direction and which is used for driving the brushless motor, and an FG magnetized portion 6b which is disposed at the lower end of the driving magnet pole portion 6a in FIG. 4 and in which magnetic poles are alternatingly formed.
In order to facilitate the understanding of the positional relationship between the driving magnet pole portion 6a magnetized on the inner peripheral face of the rotor magnet 6, and the FG magnetized portion 6b magnetized on the lower end face, the magnetization state is shown on concentric circles in FIG. 6.
A stator core 19 is disposed at a position which opposes the inner periphery of the driving magnet pole portion 6a of the rotor magnet 6. A driving coil 10 is wound on the stator core 19. An FG pattern 7 shown in FIG. 5 is formed at a position of the circuit board 2 which opposes the FG magnetized portion 6b of the rotor magnet 6.
The rotating speed of such a brushless motor is usually controlled in the following manner. The FG pattern 7 outputs a sinusoidal FG signal (see (b) of FIG. 7). The FG signal is waveform-shaped by, for example, a waveform shaper (not shown) so as to be converted into a pulse-like FG signal shown in (c) of FIG. 7. The leading edge a or the trailing edge b of the pulse-like FG is used as a trigger. Alternatively, the sinusoidal FG signal ((b) of FIG. 7) may be used as it is. In this case, the cross point a or b of the sinusoidal FG signal and AC=0 is used as a trigger. The rotational position is controlled in a similar manner. That is, the leading edge a or the trailing edge b of the pulse-like FG, or the cross point a or b of the sinusoidal FG signal and AC=0 is used as a trigger.
However, such a rotation detecting device for detecting a rotating speed or a rotational position has the following problem.
In the case where the period of the motor torque ripple (FIG. 7(a)) coincides with that of the FG signal (FIGS. 7(b) and (c)), or the case where these periods have a mutual relationship of, for example, one-to-an even number multiplication, the minimum point c of the motor torque due to the torque ripple coincides with the leading edge a and/or the trailing edge b of the pulse-like FG (in FIG. 7, both the points a and b) (naturally, the minimum point c coincides also with the cross points a and b of the sinusoidal FG signal and AC=0). This produces a problem in that the change (reaction) of the speed of the motor in response to an input of a rotating speed control signal is delayed. When such a brushless motor is used as, for example, a motor for driving a cylinder of a VTR, particularly, the delay causes undesirable phenomena such as a color slippage and disturbance in a displayed image.
This problem arises also in the case where the leading edge a or the trailing edge b of the pulse-like FG, or the cross point a or b of the sinusoidal FG signal and AC=0 is used as a trigger of a rotational position control signal. In the case, namely, the change (reaction) of the position of the motor is delayed.